The field of the present disclosure relates generally but not exclusively to optical code readers and, in a preferred application, it relates to systems, methods, and apparatus for generating virtual scan lines that compensate for the distortion in an imaging system for reading encoded symbols.
Imaging systems employing virtual scan lines have been proposed to provide a method for reading encoded symbols, such as barcode labels, by making efficient use of scanned input area. Some virtual scan line systems employ a raster line input. In one such system, input data may be provided by an area sensor, such as a two dimensional charge-coupled device (CCD) or complementary metal oxide semiconductor (CMOS) array or a video tube. In systems with a single line sensor (such as a single line CCD or a single line laser scanner) movement of the item by an operator or an additional transverse scanning mechanism, for example, may provide for the second axis of the raster scan. Preferably, not all of the scanned data, but only a select portion of the data corresponding to predefined virtual scan lines, is stored and processed.
The virtual scan line pattern may be defined based on the dimensions of barcode labels to be scanned, the size of the scanning area, and/or the characteristics of the scanning sensor and optics. The virtual scan lines are typically mapped onto the raster pattern, thereby allowing the pixels of the raster which fall on the virtual scan lines to be identified. As the raster scan proceeds, information regarding the intensity of the detected light is stored only for the pixels included in the virtual scan lines. The amount of data stored is therefore a fraction of the total data in the raster.
After the pixel values corresponding to the virtual scan line(s) have been stored, the edges of the bars and spaces crossed by the virtual scan line(s) may be determined by a suitable analog or digital edge detection method. Because only selected pixels from the sensor are processed, throughput is increased, which allows for the use of more sophisticated signal processing methods. The data can also be decoded by a decoder, which may use the scan line boundaries within the data stream to facilitate decoding. Further detail regarding the use of virtual scan lines can be found in U.S. Pat. No. 6,142,376, titled “Method and Apparatus for Reading Symbols on Items Moved by Conveyor,” U.S. Patent Application Publication No. 2006/0278708, filed Apr. 11, 2006 and titled “System and Method for Data Reading Using Raster Scanning,” and U.S. Patent Application Publication No. 2006/0081712, filed Oct. 17, 2005 and titled “System and Method of Optical Reading Employing Virtual Scan Lines.” Each of the aforementioned patent documents is hereby incorporated by reference in its entirety.
The present inventor has recognized that the effectiveness of an imaging system employing virtual scan lines may often be limited by optical distortion inherent in the optical components of an imaging system. Such optical distortion may be particularly problematic in reading bar codes or other encoded symbols with small row heights, such as, for example, the PDF417 bar code. Optical distortion may, in some circumstances, cause virtual scan lines which form a straight line trajectory to fail to intercept the entire row of the code and thereby prevent an accurate read.
For example, as shown in FIG. 1, optical distortion in an imaging system may cause an image of a bar code 10 to bow or curve near the right and left ends of the image. Accordingly, virtual scan line 50, which runs horizontally across the bottom of the image shown in FIG. 1, misses the start and stop columns of the bar code 10. Likewise, virtual scan line 60, which runs horizontally across the top of the image, misses the start and stop columns. Bar code 10 is therefore not decodable by virtual scan line 50, nor by virtual scan line 60.
The present inventor has therefore determined that it would be desirable to compensate for the distortion in the optical components of an imaging system so as to improve upon these and/or other limitations.